Improve robustness when handling LVM.
* disk/lvm.c (grub_lvm_getvalue): Return 0 when `*p' is NULL
(and leave `*p' unmodified).
(grub_lvm_iterate): Don't assume `vg->lvs != NULL' when iterating
through it.
(grub_lvm_memberlist): Don't assume `lv->vg->pvs != NULL' when
iterating through it.
(grub_lvm_open): Don't assume `vg->lvs != NULL' when iterating
through it.
(grub_lvm_scan_device): Check the return value (and fail gracefuly
when due) on each grub_lvm_getvalue() or grub_strstr() call.
Don't assume `vg->pvs != NULL' when iterating through it.
7ceeee39d7